1. Field of the Invention
The present invention relates to a light-emitting device including a microcavity structure.
2. Description of the Related Art
One of techniques for providing a full-color organic EL display is to use a white light-emitting element with a tandem structure in combination with a color filter. A tandem structure is a structure in which a plurality of light-emitting units are stacked. Note that in this specification and the like, a light-emitting unit refers to a layer or a stacked body which includes at least one region where electrons and holes injected from both ends are recombined. A light-emitting element with a tandem structure can provide high luminance with a small amount of current. For example, a light-emitting element in which two light-emitting units are stacked can provide light emission comparable to that of a light-emitting element having one light-emitting unit when current with half the density of current that flows through the light-emitting element having one light-emitting unit is made to flow through each light-emitting unit. For example, a structure in which n light-emitting units are stacked between electrodes can provide n times the luminance of one light-emitting unit without an increase in current density.
This technique of using a white light-emitting element with a tandem structure in combination with a color filter has the following advantage: it is not necessary to provide separate light-emitting layers for respective sub-pixels (e.g., three sub-pixels of R, G, and B), which leads to high yield and easy manufacture of high-resolution displays. Moreover, it is possible to improve the color purity of light emitted from each sub-pixel by applying a microcavity structure to a pixel provided with the white light-emitting element and the color filter.
In particular, a microcavity structure can be formed relatively easily in a top-emission light-emitting element in such a manner that a reflective electrode, an EL layer, and a semi-transmissive and semi-reflective electrode are formed in that order from the substrate side and a transparent electrode is formed as an optical adjustment layer between the reflective electrode and the EL layer. FIG. 6 illustrates a structural example of a pixel that is provided with a light-emitting element with a microcavity structure and a color filter.
A conventional light-emitting device in FIG. 6 includes light-emitting elements in each of which a reflective electrode 501, a transparent electrode 502, an EL layer 506, and a semi-transmissive and semi-reflective electrode 507 are stacked in that order. Over the light-emitting elements, a red color filter (CF Red), a green color filter (CF Green), and a blue color filter (CF Blue) are provided.
The EL layer 506 has a structure in which a first light-emitting unit 508 including a first light-emitting layer 503, an intermediate layer 509, and a second light-emitting unit 510 including a second light-emitting layer 504 and a third light-emitting layer 505 are stacked in that order. The first light-emitting layer 503 emits blue light, the second light-emitting layer 504 emits green light, and the third light-emitting layer 505 emits red light. The intermediate layer 509 preferably has a structure in which, for example, an electron-injection buffer layer, an electron-relay layer, and a charge generation layer are stacked in that order from the anode side.
The above light-emitting device includes a microcavity structure. An optical length L between the reflective electrode 501 serving as a reflecting mirror and the semi-transmissive and semi-reflective electrode 507 is adjusted so that light emitted from the EL layer 506 is repeatedly reflected between the reflective electrode 501 and the semi-transmissive and semi-reflective electrode 507; thus, light with a specific wavelength can be selectively amplified and emitted outside.
To perform full-color display with the above light-emitting device, light with wavelengths for three colors, red (R), green (G), and blue (B), is to be amplified, for example, in one display panel. Accordingly, light of colors that correspond to three sub-pixels of R, G, and B needs to be amplified. To achieve that, the thickness of the transparent electrode 502 is varied to provide optical lengths L corresponding to the three wavelengths for R, G, and B (e.g., see Patent Document 1).